1. Field
Embodiments described in this specification relate to a semiconductor memory device.
2. Description of the Related Art
In recent years, resistance varying memory devices employing a variable resistor as a memory element are attracting attention as candidates to succeed flash memory. The resistance varying memory devices are assumed here to include not only the narrowly-defined resistance varying memory (ReRAM: Resistive RAM), in which a transition metal oxide is used as a recording layer to store a resistance state of the transition metal oxide in a non-volatile manner, but also the likes of phase change memory (PCRAM: Phase Change RAM), in which chalcogenide or the like is used as a recording layer to utilize resistance information of a crystalline state (conductor) and an amorphous state (insulator).
There are known to be two kinds of operation modes in memory cells of a resistance varying memory device. In one, referred to as bipolar type, the polarity of applied voltage is switched to set a high-resistance state and a low-resistance state. In the other, referred to as unipolar type, the voltage value and voltage application time are controlled, thus allowing the high-resistance state and the low-resistance state to be set without switching the polarity of applied voltage.
In the case of unipolar type ReRAM, write of data to the memory cell is performed by applying a certain voltage to the variable resistor for a short time. This causes the variable resistor to change from the high-resistance state to the low-resistance state. This operation to change the variable resistor from the high-resistance state to the low-resistance state is hereinafter referred to as a setting operation. On the other hand, erase of data in the memory cell is performed by applying a certain voltage to the variable resistor that is in the low-resistance state after the setting operation for a long time. The certain voltage applied during the erase operation is lower than that applied during the setting operation. This causes the variable resistor to change from the low-resistance state to the high-resistance state. This operation to change the variable resistor from the low-resistance state to the high-resistance state is hereinafter referred to as a resetting operation. The memory cell adopts, for example, the high-resistance state as a stable state (reset state), and, in the case of binary data storage, write of data is performed by the setting operation in which the reset state is changed to the low-resistance state.
When the setting operation and resetting operation are executed on the resistance varying memory device, a pulse generated in a pulse circuit and having a certain voltage value and pulse width (pulse application time) is applied to a selected memory cell. If these setting operation and resetting operation on the resistance varying memory device are repeated several times, it becomes gradually more difficult for the resistance state of the memory cell to change. In particular, there is a risk of resetting failure occurring, in which the memory cell stops changing to the high-resistance state even if it is applied with a resetting pulse. A memory cell in which resetting failure has occurred does not function as a memory element if left as is. Hence measures are required against resetting failure to prevent the resetting failure from affecting operation of the resistance varying memory device.